Mono- N-alkyl-dinitroalkylanilines such as N-sec-butyl-4-tert-butyl-2,6-dinitroaniline [(1,1-dimethylethyl)-N-[1-methylpropyl)-2,6,dinitrobenzenamine; butralin] are known selective herbicides generally used for pre-emergence control of annual broad-leaved weeds and grasses in cotton, soybeans, rice, barley, beans, alliums, vines, ornamentals and orchards of fruit and nut trees. They are also used to control suckers of tobacco.
The synthesis of N-sec-butyl-4-tert-butyl-2,6-dinitroaniline is generally described in a number of U.S. Patents including, for example, U.S. Pat. Nos. 3,672,866; 3,927,127 and 3,991,116 all to Damiano. The process described in U.S. Pat. Nos. 3,672,866 and 3,991,116 begins with a starting material, 4-tert-butylphenol. The 4-tert-butylphenol is subjected to nitration utilizing nitric acid in acetic acid solution to produce 2,6-dinitro-4-tert-butylphenol. The 2,6-dinitro-4-tert-butylphenol is then chlorinated to produce 2,6-dinitro-4-tert-butylchlorobenzene. This 2,6-dinitro-4-tert-butylchlorobenzene is then converted into N-sec-butyl-4-tert-butyl-2,6-dinitroaniline by amination with sec-butylamine.
While this process is effective in producing the desired product, it does suffer from a number of distinct disadvantages. First and foremost is the relatively low yield of the nitration step: only approximately 70%. Second, the process necessitates purification of the 2,6-dinitro-4-tert-butylphenol product of the nitration step by crystallization from large volumes of highly volatile and flammable hexane. This is both hazardous and time consuming. Third, is the necessity of utilizing thionyl chloride in high boiling and expensive solvents for the chlorination step. This step disadvantageously leads to the equimolar formation of toxic and environmentally hazardous gaseous by-products; hydrochloric acid and sulfur dioxide. Further, the slow fifteen hour process also necessitates further purification of the crude product (by crystallization) from hexane thereby further elevating the costs of production. Fourth, the amination step requires seven hours and the use of excessive amounts of sec-butylamine while unfortunately being accompanied by the formation of equimolar quantities of sec-butylamine hydrochloride as a by-product.
The process described in U.S. Pat. No. 3,927,127 begins with a starting material, 4-tert-butylchlorobenzene. The 4-tert-butylchlorobenzene is subjected to nitration utilizing a mixture of nitric and sulfuric acids to produce 2,6-dinitro-4-tert-butylchlorobenzene. The 2,6-dinitro-4-tert-butylchlorobenzene is then converted to N-sec-butyl-4-tert-butyl-2,6-dinitroaniline by amination with sec-butylamine. While the process is effective in producing the desired product, it does suffer from a number of distinct shortcomings. First and foremost, it is necessary to use a highly concentrated nitric and sulfuric acid mixture at elevated temperatures for a prolonged time to complete the nitration step of the synthesis. This unfortunately involves a substantial risk of explosion. Second, it is necessary to use an excess of sec-butylamine because of the equimolar formation of a sec-butylamine hydrochloride as a by-product in the third stage of the process. This, unfortunately, necessitates the application of a high boiling point solvent (e.g. xylene) to allow filtration separation of the by-product thereby producing a number of additional safety hazards and environmental concerns.
An alternative synthesis for N-sec-butyl-4-tert-butyl-2,6-dinitroaniline is set forth in U.S. Pat. Nos. 4,289,907 and 4,395,572 to Chan. The starting material in this synthesis is a nitrophenol such as 2,6-dinitro-4-tert-butylphenol. The nitrophenol is methylated by reaction with methyl iodide/potassium carbonate to produce 4-tert-butyl-2,6-dinitroanisole. The 4-tert-butyl-2,6-dinitroanisole is then reacted with sec-butylamine to produce N-sec-butyl-4-tert-butyl-2,6-dinitroaniline. While effective in producing the desired product, this process also suffers from a number of distinct disadvantages.
First, the process involves the use of methyl iodide. This material is expensive, volatile, carcinogenic and poisonous. The resulting health and environmental concerns require the establishment of special handling procedures.
Second, the resulting yield of the process is only approximately 71%. Third, the product is accompanied by the formation of bulk amounts of gaseous, liquid and solid waste including the carcinogenic methyl iodide starting material. Fourth, the process involves the use of excess amounts of potassium carbonate and highly flammable and volatile acetone as a solvent. This adds to the expenses and handling concerns of the process.
From the above description it should be appreciated that a need exists for an improved process for producing N-alkyl-dinitroalkylanilines in a more economical and environmentally safer manner.